Electrical power systems are nonlinear systems that are large and quite complex. Their limited available control and generally sluggish dynamic response complicate these power systems. Moreover, the magnitude of these power systems does not easily provide for a rapid control response to disturbances. Restoring a system to its pre-disturbance level can take hours. Furthermore, as a result of the increasing social and environmental costs of installing transmission lines, transmission systems will be driven closer to their limits. The difficulty in controlling these power systems will only increase with their heavy loads.
Power systems in the United States are already being pushed more heavily now than ever before. Recent events on the West Coast demonstrate potential nationwide problems. The United States' power transmission infrastructure is encountering a variety of new demands that it was never designed to meet. At the same time, major componentry of the systems are nearing the end of their design lifetime. A major investment in rebuilding the power transmission infrastructure will be required, as well as the development of new technologies, in order to get the maximum utilization out of the existing plants at a lower cost.
Power flow from one point to another within an interconnected system obeys Kirchhoff's Laws. Current is divided between parallel pathways leading to the identical destination. Unfortunately, pathways running parallel to the desired paths may be owned or operated by separate entities, or may already be loaded to capacity. Control of the loop flows is presently accomplished through the use of phase shifting transformers and series capacitors. These devices lack the ability to be rapidly adjusted, a significant detriment in a field where flows of power may quickly be altered. The tremendous increase in the use of retail wheeling and contracted power transfers requires a substantially increased ability to determine the pathways taken by power flows.
Fuel cells have been known for over 150 years, and are currently positioned to make substantial contributions in the field of stationary power generation. Ludwig Mond and Charles Langer, who attempted to build the first practical fuel cell device using air and industrial coal gas, coined the term “fuel cell.” Early attempts to build fuel cells for converting coal or carbon directly into electricity failed as a result of a dearth of knowledge regarding materials and the kinetics of electrodes.
The first successful fuel cell devices resulted from inventions that improved on the previously employed expensive platinum catalysts with a hydrogen-oxygen cell utilizing a less corrosive alkaline electrolyte and inexpensive nickel electrodes. However, the technical challenges were discouraging and it was not until the 1950's that fuel cell systems showed promise as energy sources with significant output. At that time, the National Aeronautics and Space Administration (NASA) turned to fuel cells for compact electricity generators to provide onboard power for manned space missions.
Generally, fuel cell devices produce electricity by combining hydrogen ions that are derived from a hydrogen-containing fuel with oxygen atoms. Unlike batteries, which provide the fuel and oxidizer internally and must be recharged periodically, fuel cells utilize a supply of ingredients from an outside source and produce power so long as the fuel supply is maintained. By continuously changing the chemical energy of a fuel source, such as hydrogen gas, and oxidant, such as oxygen or air, to electrical energy, a typical fuel cell device generates electricity. This process does not consume the fuel to produce heat; hence the thermodynamic limits on efficiency are much higher than the traditional power generation processes. A fuel cell generally consists of two catalytic electrodes separated by an ion-conducting membrane. The hydrogen fuel is ionized on one electrode, and the subsequent hydrogen ions diffuse across the membrane to interact with the oxygen ions on the surface of the other electrode. If current flow is prohibited from one electrode to the other, a potential gradient develops, stopping the diffusion of the hydrogen ions. Permitting current to flow from one electrode to the other through an external load creates power.
The membrane that separates the electrodes ideally provides for the diffusion of ions from one electrode to the other, and additionally keeps the fuel and oxidant gases apart. The membrane prevents the flow of electrons, as well as the diffusion of the fuel or oxidant gases, to reduce the possibility of explosions and other unintended consequences. If electrons pass through the membrane, the device is shorted out, thus eliminating or reducing the useful power formed by the fuel cell.
A fuel cell having catalytic electrodes in close contact with the membrane material reduces the contact resistance that occurs when the ions move between the catalytic electrode and the membrane. The aforementioned close contact can be accomplished by incorporating the membrane material into the electrodes.
The fuel cell facilitates chemical reactions that produce either hydrogen- or oxygen-bearing ions at one of the electrodes of the cell. The ions then pass through an electrolyte, such as phosphoric acid or carbonate, and react with oxygen atoms. This interaction results in an electric current at both electrodes, and produces heat and water vapor as waste products. The strength of the electric current is proportional to the surface area of the electrodes. The voltage of a fuel cell is limited electrochemically to approximately 1.23 volts per electrode pair. Thus, fuel cells then can be stacked until the desired power level is reached.
One of the major challenges in developing practical applications for fuel cells has been to improve the economics through the use of low-cost components with acceptable component life and performance. As a result, fuel cells are distinguishable by the type of electrolyte used. In the realm of stationary power generation, the conventional types of fuel cells use phosphoric acid, carbonate, or solid oxide as electrolytes. Differentiation between fuel cell approaches is found in the type of electrolyte used.
The phosphoric acid approach is the most established of the approaches. Platinum is required as a catalyst for the electrodes. Conversion of the natural gas, known as reforming, used as fuel to a hydrogen-rich gas the system requires occurs outside the fuel cell stacks. The system complexity of this approach yields capital costs that are higher and efficiencies that are lower than those for the two other approaches.
The carbonate approach operates at higher temperatures, at or slightly above ambient pressure, and uses less expensive, nickel-based electrodes than the phosphoric acid approach. Reforming can occur inside the fuel cell stacks. The major difficulties with carbonate technology include (1) the complexity of working with a liquid electrolyte rather than a solid and (2) the carbonate ions from the electrolyte are used up in the reactions at the anode, making it necessary to compensate by injecting carbon dioxide at the cathode.
The solid oxide approach is the least developed of these approaches. It uses a coated zirconia ceramic as the electrolyte. The electrochemical conversion process occurs at extremely high temperatures, sustaining internal reforming. The fuel cells may be either flat plates or tubular in shape. Unresolved manufacturing difficulties with all-ceramic construction in mass producing these fuel cells hampers this approach.
Additionally, a proton exchange membrane, also known as a polymer electrolyte membrane, fuel cell approach has been postulated for submegawatt stationary power plant applications. The proton exchange membrane allows protons to flow through, but prohibits the passage of electrons. As a result, while the electrons flow through an external circuit, the hydrogen ions flow directly through the proton exchange membrane to the cathode, where they combine with oxygen molecules and the electrons to form water. A fuel cell under this approach operates at 175° F., uses a platinum catalyst, and is susceptible to poisoning by carbon monoxide and other impurities.
It is thus an object of the present invention to provide a fuel cell energy plant that has improved efficiency through a combination of a fuel cell portion and an electrolysis portion. The fuel cell portion combines hydrogen and oxygen to produce electricity, forming water as a waste product that is then split by the electrolysis potion to form hydrogen and oxygen. The hydrogen and oxygen is sent back to the fuel cells for reuse.
Therefore, a continuing need exists for a fuel cell energy plant that overcomes the efficiency problems existing in prior fuel cell systems, including but not limited to fuel usage, by coupling a fuel cell portion to an electrolysis portion.